This invention relates to a catalyst useful for the selective hydrocracking of fractionated heavy reformate containing alkyl aromatics into more useful compounds. More specifically, this invention is concerned with a catalyst for a selective hydrocracking or scission process whereby a fractionated heavy reformate stream comprising alkylbenzenes having substituted thereon methyl, ethyl, isopropyl and butyl radicals is converted primarily to methyl-substituted benzenes and formation of benzene is minimized utilizing a catalyst comprising highly purified gamma or eta alumina containing essentially no silica, a surface area of at least 100 m.sup.2 /g, and rhodium and zinc. Fractionated heavy reformates can be reformates from which C.sub.7 aromatics and lighter components have been largely removed. This stream typically can contain C.sub.8 -C.sub.10 aromatics consisting of xylenes, ethylbenzene, isopropyl benzene, butyl benzene and dimethyl ethyl benzene. Alkyl scission of the alkylbenzenes having substituted thereon alkyl radicals of 1 to 4 carbon atoms in this fraction to methyl substituted benzenes would provide an aromatic stream which is ideally suited for preparing xylenes via transalkylation.
In the prior art, methods which have been used to produce aromatic chemicals from fractionated heavy reformates utilize a hydrocracking and/or a hydrodealkylation step to convert the C.sub.9 and C.sub.10 + aromatic components to benzene, toluene and C.sub.8 aromatics. The C.sub.6 + paraffins are converted into readily distillable low boiling hydrocarbons of C.sub.5 and lighter. Processes utilizing these principles are described in U.S. Pat. Nos. 3,957,621 and 3,862,254.
Other typical prior art on hydrodealkylation of alkyl aromatics is the following:
U.S. Pat. No. 2,422,673 teaches hydrodealkylation or demethylation of an alkyl aromatic using a catalyst containing nickel or cobalt on diatomaceous earth. Temperatures used in the process are between 350.degree.-650.degree. F. and pressures are between subatmospheric to 1000 psig. The reaction is carried out at a low pressure of hydrogen so as to obtain a high proportion of demethylation and a relatively small amount of hydrogenation of aromatic hydrocarbons to naphthenic hydrocarbons.
U.S. Pat. No. 2,734,929 teaches hydrodealkylation of alkyl aromatics. Ethylbenzene is hydrocracked to principally benzene and xylene to toluene. The catalyst contains a Group VIB or Group VIII metal hydrogenation component such as chromium, molybdenum, tungsten, iron, cobalt, ruthenium, rhodium, etc. The catalyst is preferably suspended on a carrier which has no adverse effect on the reaction. Gel alumina, which contains silica and which usually has a surface area of over 100 m.sup.2 /g, as measured by gas adsorption, is preferred. The process requires a gaseous diluent which, column 2, lines 16-18, is stated as being a critical feature of the invention. Operating conditions include a temperature between 800.degree.-1500.degree. F. and a pressure of 0-5000 psig.
U.S. Pat. No. 3,478,120 discloses a process for hydrodealkylation of ethylbenzene to toluene, benzene, methane and ethane with the hydrodealkylation being carried out in the presence of xylenes. The catalyst used comprises an iron group metal on calcium aluminate. Operating conditions include a temperature range of 500.degree.-1200.degree. F. and pressure from atmospheric to 2000 psig.
U.S. Pat. No. 3,306,944 teaches catalytic hydrodealkylation of alkyl aromatic hydrocarbons. Examples are cumene to ethylbenzene (predominantly) and toluene; p-t-butyltoluene to p-propyltoluene, p-ethyltoluene and xylene. The catalyst comprises a metal selected from the group consisting of rhodium, ruthenium, etc. upon a promoted metal oxide support. "Promoted" refers to pretreatment of the support with a salt or hydroxide of an alkali metal or alkaline earth metal. The preferred metal oxide support is gamma alumina which has a surface area ranging from 100 to about 300 m.sup.2 /g and is freed from combined or adsorbed water.
U.S. Pat. No. 3,992,468 teaches a catalytic hydrodealkylation process of alkyl aromatic hydrocarbons to benzene. The catalyst comprises at least two metals, one selected from the group consisting of ruthenium, cobalt, osmium, palladium, rhodium, iridium, platinum, chromium, molybdenum, tungsten and manganese, the other selected from, among others, zinc, cadmium, and gallium, the final catalyst having a specific surface area of from 1 to 100 m.sup.2 /g. The carrier is of low acidity and can be alumina, including gamma alumina, magnesia, magnesia-silica, acidic alumina, alumina-silica, among others, including molecular sieves.
U.S. Pat. No. 3,975,454 teaches a catalytic hydrodealkylation process of alkyl aromatic hydrocarbons at a temperature within the range of 250.degree.-400.degree. C. The catalyst comprises the compounds formed from either graphite and an alkali metal, or graphite, an alkali metal and a compound of a metal selected from the group consisting of Group VIII of the Periodic Table which includes iron, nickel, cobalt, etc. Surface area of the catalyst of Example I was cited as about 20 m.sup.2 /g.
Accordingly, the prior art teaches catalytic hydrocracking and/or hydrodealkylation of alkyl aromatic hydrocarbons. However, the catalytic selective hydrocracking or alkyl scission of alkyl aromatic hydrocarbons is not taught wherein the hydrocarbon stream comprises C.sub.8 -C.sub.10 aromatic hydrocarbons and the catalyst used comprises high-surface area, highly purified gamma or eta alumina containing essentially no silica, with a surface area greater than 100 m.sup.2 /g, and rhodium and zinc under process conditions which selectively scission alkylbenzenes to predominantly methyl-substituted benzenes.